The invention is useful in the field of traffic surveillance radars.
In certain multitarget situations when traffic is being observed by a police doppler radar in the same lane as the patrol car and in the opposite lane, it is useful to be able to limit the targets whose speeds are displayed to targets only in the same lane or only in the opposite lane. The same is true for stationary operation at the roadside. Further, it is useful to be able to use the radar to search for only the strongest target or both the strongest target and the fastest target or only the fastest target.
Police doppler fast fourier transform digital signal processing radars which can find the strongest target ha e been in public use for many years starting with the introduction of the STALKER(trademark) handheld, digital doppler digital signal processing (DSP) radar by Applied Concepts, Inc. of Plano, Tex. Police doppler fast fourier transform digital signal processing radars which can find the strongest target or the strongest target with simultaneous display of the fastest target have been in public use for many years starting with the introduction of the STALKER DUAL(trademark) dash mounted doppler police traffic surveillance by Applied Concepts, Inc. of Plano, Tex. The STALKER DUAL(trademark) dash mounted doppler police traffic surveillance radar is fully described in U.S. Pat. No. 5,691,724 which has an effective filing date of Feb. 10, 1995, the full contents of which are hereby incorporated by reference. Police doppler fast fourier transform digital signal processing radars which can find the strongest target or the fastest target in separate searches selected by the operator have been in public use for many years starting with the introduction of the EAGLE(trademark) dash mounted, digital doppler DSP series of radars by Kustom Signals, Inc. of Lenexa, Kans. The EAGLE(trademark) dash mounted, digital doppler DSP series of radars are partially described in U.S. Pat. No. 5,528,246, the contents of which are hereby incorporated by reference. There is a significant difference between the STALKER DUAL(trademark) dash mounted doppler police traffic surveillance radar and the EAGLE(trademark) dash mounted, digital doppler DSP series of radars, as described in U.S. Pat. No. 5,528,246 in that in the STALKER DUAL(trademark) dash mounted doppler police traffic surveillance radars, there is no separate search for the fastest target which can be selected by the operator and which ignores the strongest targets. The STALKER DUAL(trademark) dash mounted doppler police traffic surveillance radar always searches for the strongest target even when the operator selects fastest mode, and will not search for a fastest target until a valid strongest target has been found.
All of the FFTs done in the STALKER(trademark) handheld, digital doppler digital signal processing (DSP) radar and EAGLE(trademark) dash mounted, digital doppler DSP series of radars are non-complex FFTs, anti none of these radars had direction sensing capability. An analog doppler radar that has direction sensing capability is disclosed in U.S. Pat. No. 5,557,281, the contents of which are hereby incorporated by reference. That radar discloses a quadrature shifted two channel doppler signal that is processed by a PLL to lock onto the frequency of one doppler signals in one channel. Another set of circuits determines direction by examining both channels and sends a signal to the PLL line of circuitry to tell it which direction the target it is locked on is travelling. It is unclear if this analog radar is operative in a multitarget environment where the strength of the various targets is varying rapidly since it is unclear which target the PLL will lock onto and which target will be sensed for direction. It is possible that the direction sensing circuitry will tell the PLL circuitry the direction of a different target than the target to which the PLL is locked thereby causing an error.
Therefore, a need has arisen for a digital FFT radar with direction sensing capability. Further, a need has arisen for such a radar that can find the strongest or fastest and strongest target or the fastest target alone.
There is disclosed herein a digital, complex FFT radar with direction sensing capability using a two channel doppler front end with a 90 degree phase shift in the doppler signals of the two channels. The two channels of information are, optionally, digitally gain controlled, and are digitized. The digital samples from each channel are then processed by a digital signal processor using a complex FFT resulting in a receding target spectrum and an approaching target spectrum of Fourier components.
Several single mode radars are disclosed in the form of flowcharts indicating the manner of processing the two half spectra of receding and approaching targets to find either the strongest target alone or the fastest target alone in various stationary, moving same lane or moving opposite lane operation.
There is also disclosed a process for a multimode digital FFT, direction sensing, doppler radar where the operator can select between the following modes:
stationary, strongest only, receding only
stationary, strongest only, approaching only
stationary, strongest and fastest, approaching only
stationary, strongest and fastest, receding only
moving, same lane only, strongest only
moving, same lane only, strongest and fastest
moving, opposite lane only, strongest only
moving, opposite lane only, strongest and fastest
An alternative embodiment is also disclosed for a multimode digital FFT, direction sensing, doppler radar where the operator can select between the following modes:
stationary, strongest only, receding only
stationary, strongest only, approaching only
stationary, fastest only, approaching only
stationary, fastest only, receding only
moving, same lane only, strongest only
moving, same lane only, fastest only
moving, opposite lane only, strongest only
moving, opposite lane only, fastest only
In all fastest search embodiments, it is preferred to collect samples which were digitized at a known gain level by virtue of using the DSP to control the gain of an amplifier such as the amplifiers 62 and 64 in FIG. 3 and the two amplifiers 62 and 64 in the two channels of FIG. 4. Further, in all fastest searches, it is preferable to do a strongest search first and to maintain a record of some number of the strongest signals in the spectrum such that the fastest target candidates can be screened to eliminate false fastest targets. It is known that false fastest targets can arise as second or third harmonics of patrol speed or other strong signals in the spectrum and can arise as the sum products of two strong signals in the spectrum if the signals are strong enough to exceed a harmonic threshold or if the signals are strong enough to exceed an intermodulation product threshold. By doing a strongest search first and keeping track of the frequencies of the strong signals, qualification tests can eliminate fastest candidates if it appears that they may be harmonics or intermodulation products of the known strong signals. It is necessary to know the frequency of the patrol speed and strong signals to make such a screening. Fastest target processing allows the operator to xe2x80x9clook pastxe2x80x9d the strongest target (usually the closes vehicle) and measure and display speeds of more distant vehicles which are approaching at a faster speed than the strongest target. Intermodulation effects can produce spectral signals which can be falsely interpreted as weak xe2x80x9cfasterxe2x80x9d targets. In order to determine if a target is a rue faster target, the received data is operated on by a 512 point FFT, which results in a 256 point spectral analysis. The spectral strength and spectral bin number of the five strongest spectral lines are used to determine if the potential target bin number is a multiple of one of these line or if its spectral bin number is nearly the same as the sum of any two of the five lines or nearly the same as the absolute difference of any two of the five lines. Thus, the patentee adopts as a definition of the xe2x80x9cfastest targetxe2x80x9d for purposes of this specification and its appended claims, a target which is faster than the strongest target and which has been qualified to make sure it is not a false fastest target.
A further degree of refinement in the fastest target screening process is provided by using the controlled gain amplifiers to amplify the doppler signals before they are digitized. By knowing the gain that was in effect as each batch of samples were gathered, it is possible to calculate the true power of any signal in the spectrum from its apparent or relative power and the gain that was in effect when the samples were collected. This allows fastest target candidates to be not rejected even if they are at a frequency that is a double or triple of the patrol speed or a strong signal if the patrol speed or strong signal does not have a true power that exceeds an experimentally determined harmonic generation threshold. Likewise, a fastest candidate that has a frequency that happens to be at the sum of the frequencies of two strong signals need not be eliminated if the true powers of the two strong signals do not exceed power thresholds which are experimentally determined to be likely to cause intermodulation products to exist. This has the significant advantage that it does not blind the radar to legitimate fastest targets if the underlying strong signals are not strong enough to have caused harmonics or intermodulation products.